1. Field of the Invention
The present invention relates to a process for the production of a trimellitic acid by oxidizing dialkyl aromatic aldehyde and/or its oxide derivative in a liquid phase, and a process for the production of high-quality trimellitic acid anhydride from the trimellitic acid.
2. Description of the Prior Art
Conventionally, aromatic polycarboxylic acid have been produced by oxidation of polyalkylbenzene, and trimellitic acid from pseudocumene, trimesic acid from mesitylene, pyromellitic acid from durene and mellophanic acid from isodurene are known. For example, JP-A-6-16655 discloses a process for the production of trimellitic acid by oxidizing pseudocumene in a liquid phase, in which the oxidation is carried out in the presence of cerium, cobalt and manganese and optionally in the co-presence of zirconium and a bromine catalyst and cerium and bromine are allowed to react in a second oxidation step by a batch method. However, since the above catalyst system is complicated, and further, since the reaction method is complicated, it is difficult to recover and recycle the catalysts.
In the oxidation of alkylbenzenes to corresponding aromatic polycarboxylic acids, the alkylbenzenes differ from one another in reactivity depending upon the position of substituted methyl group. Trimellitic acid or pyromellitic acid formed from pseudocumene or durene has a structure in which one carboxylic acid is positioned in o-position to the other, and therefore, a heavy metal catalyst is decreased in activity and the yield from the oxidation decreases as compared with polymethylbenzene having such a structure. Various proposals have been therefore made for the improvement of a catalyst system. Yields from the oxidation are improved, while it is difficult to recover and recycle catalysts since the catalyst system is complicated.
On the other hand, it is known that an aromatic aldehyde is formed by converting an aromatic hydrocarbon to a formyl compound and used as a raw material for producing an aromatic polycarboxylic acid. For example, JP-B-58-2222 discloses a method in which an aromatic aldehyde is oxidized in a water solvent to continuously produce an aromatic polycarboxylic acid. This method requires a special reactor material formed of zirconium due to a high bromine concentration.
Further, JP-A-57-38745 discloses a method in which a polyalkyl aromatic aldehyde is oxidized in the presence of cobalt, manganese and bromine in a acetic acid solvent, to produce an aromatic polycarboxylic acid.
Plastic industries use trimellitic acid as an intermediate for the synthesis of a resin or a plasticizer. In many cases, trimellitic acid is converted to trimellitic acid anhydride by dehydration, and in particular, it is used as an intermediate for the production of a polyester. It is therefore strongly desired to supply an inexpensive trimellitic acid.
The present inventors have studied the production of trimellitic acid by oxidizing a dialkyl aromatic aldehyde in the presence of cobalt, manganese and bromine in an acetic acid solvent according to the method of the above JP-A-57-38745, and have found the following. It is difficult to recover and recycle the catalysts due to a large metal content in a crystal, and it is also required to add improvements in view of yields.
It is a first object of the present invention to provide a process for the production of trimellitic acid, which does not require any special reactor material for continuously oxidizing a dialkyl aromatic aldehyde and/or its oxide derivative, which permits the recycle of a catalyst and which provides high yields.
In a method using a catalyst system of cobalt, manganese and bromine like JP-A-57-38745, it is desired to improve the reaction rate and the yield.
It is a second object of the present invention to provide a process for the continuous and advantageous production of trimellitic acid from a dialkyl aromatic aldehyde and/or its oxide derivative as raw material in the presence of a highly active catalyst developed in place of the catalyst system of cobalt, manganese and bromine.
Trimellitic acid is an aromatic tri-basic acid and is essential as a raw material for a high-grade plasticizer or a heat-resistant plastic. A plasticizer or a plastic requires a less-colored high-purity trimellitic acid anhydride as a raw material therefor.
As described in JP-B-58-2222 and JP-A-61-280448, trimellitic acid anhydride is produced from crude trimellitic acid obtained by oxidizing pseudocumene, dimethylbenzaldehyde or its oxide intermediate with molecular oxygen in the presence of bromide ion or in the presence of bromide ion and heavy metal ion as a catalyst.
Trimellitic acid anhydride is produced by thermally dehydrating crude trimellitic acid at 220 to 230.degree. C. and distilling the resultant trimellitic acid anhydride under reduced pressure, as is described in Ullman's literature ("Encyclopedia of Industrial Chemistry", 4th ed. vol. 9, page 150).
However, the trimellitic acid anhydride produced by the above method is not necessarily satisfactory in hue and purity, and it is desired to supply trimellitic acid anhydride having a higher quality as a raw material for a high-grade plasticizer or heat-resistant plastic in recent years.
JP-B-47-35421 discloses a method in which trimellitic acid is treated with boric acid. Further, For an improvement of this method, Japanese PCT Publication No. 4-501271 discloses a method in which at least 0.1% by weight of boron is added to a trimellitic acid anhydride, the mixture is thermally treated and the treated trimellitic acid anhydride is distilled for improving its hue.
However, the trimellitic acid anhydride produced by the above method is not yet necessarily satisfactory in hue and purity, and it is further desired to supply trimellitic acid anhydride having a much higher quality as a raw material for a high-grade plasticizer or heat-resistant plastic in recent years.
Further, when pseudocumene or dimethylbenzaldehyde as a raw material contains impurities which cannot be converted to trimellitic acid by oxidation, the resultant trimellitic acid anhydride is degraded in hue and purity in some cases, and it is therefore desired to develop a method of producing trimellitic acid anhydride free of an influence caused by the raw material.
It is a third object of the present invention to provide a process for the industrially advantageous production of a high-quality trimellitic acid anhydride from a crude trimellitic acid having a low purity and containing coloring substances, obtained by the oxidation of pseudocumene or dimethylbenzaldehyde.